The invention uses various materials which are electrically either conductive, insulating or semiconducting, although the completed semiconductor circuit device itself is usually referred to as a "semiconductor". One of the materials used is silicon, which is used as either single crystal silicon or as polycrystalline silicon material, referred to as polysilicon or "poly" in this disclosure. The invention refers to a method of controlling memory devices which include a page mode for faster access, and is not restricted to implementations which involve semiconductor devices.
Computer systems, whether general purpose, desktop, work stations, or imbedded controllers (e.g., small computers used to control tools, instruments, or appliances), all include some form of read/write memory. At the present time, the most popular form of read/write memory is the semiconductor DRAM (or Dynamic Random Access Memory). The DRAM is so popular because it offers the best available combination of performance, density, and price; but it does have some disadvantages. Since its memory is based on the storage of an electrical charge on a small capacitor, the charge on the capacitor must be periodically renewed to prevent it from "forgetting" (this process of renewing the charge is called refresh). Internally, the memory cells are organized into a rectangular array where each cell is addressed as being in a particular row, and at a particular column location within the row.
The most common DRAMs minimize the package size by using the same input pins to select the row and the column, i.e., multiplexing the addresses. To read (or write) to a particular cell, the row address is presented on the address pins and a control signal (called RAS) is taken low; then the column address is presented on the same address pins and while RAS is kept low, another control signal (called CAS) is taken low. When RAS first goes low, sensing circuits within the memory device measure the charge on the capacitors for every cell that is a part of the specified row. When CAS goes low, the resulting data from the cells within that row as specified by the column address is transferred to the output pin(s). The time from RAS going low until data is available at the output (for a read operation) is called the "row access time" and is typically 100-120ns. The time from CAS going low until data is available is approximately half of the row access time and is called the "column access time" (some DRAM manufacturers measure column access time from the time the column address is put onto the address pins instead of when CAS goes low).
The maximum operating speed of a computer device depends on two things: how fast the processor can execute instructions, and how long it has to wait for instructions or data. The maximum operating rate will occur when the read/write memory is sufficiently fast that the processor never has to wait for instructions or data. This is referred to as "operating with zero wait states". Since many of the most common processors require memory accesses of under 100 ns (commonly 50-60ns), using a DRAM memory with full RAS/CAS cycles would require slowing down the processor by adding "wait states". To operate without wait states either requires a faster read/write memory device or some means must be found to speed up the DRAM memory. One memory device that can be used is the SRAM (Static Random Access Memory), which has access times as quick as one-tenth that of the DRAM's. However, that faster access time comes at the expense of a more complex device which means that it cost more, and is less dense than the DRAM.
Several different modifications to the DRAM have been devised to reduce the access time. The most common modification for general read/write applications is the "page mode". In the page mode, once RAS has been taken low (which effectively remembers the row address), any cell on that row (referred to as that "page") can be accessed by presenting its column address and taking CAS low. If RAS is kept low, CAS can be taken high, another column address presented and CAS taken low again to access another cell within the page. This process of presenting a new column address and taking CAS low can be repeated several times; and in each case the access time from the new column address to data is now the column access time instead of the longer row access time and is typically less than 60 ns.
The logic required to use the DRAM parts in this manner is fairly complex. There is a limit to how long RAS can be held low without taking it back high. Also, DRAMs need to be refreshed at regular intervals, so the page mode may need to be interrupted to perform a refresh cycle. For the second, and following page mode cycles, it is necessary to compare the current row address with the previous address to determine if it is the same; if not, then a penalty is imposed because RAS has to be taken high for a specified period of time (about 100 ns) before the new row address can be input. After RAS goes back low, then the column address can be presented and CAS taken low just as previously described for a normal access. That means that if the new request is in a different row, that access will actually be slower than for a standard DRAM access. Therefore, an optimal controller for page mode operation of main DRAM memory would include some means of automatically switching between page mode and non-page mode of DRAM operation depending on some prediction of whether the next access will be on the same page or on another page.
A DRAM controller can operate DRAMs in a page mode of operation by holding RAS low. As mentioned, this reduces access time for successive memory accesses, but only true if those accesses are, in fact, addressed on the same page of the DRAM. If the access time in normal mode is 120 ns, then page mode access time would be in the range of approximately 50 or 60 ns. If the address turns out to be on a different page, then RAS must be sequenced, followed by CAS, which is now delayed. Consequently, a "miss", where a sequential memory address is not in the same row, results in increased access time, which would be, in the example, in the range of 220 ns. For this reason, it is advantageous for a page mode controller to be "clairvoyant" in predicting when sequential addresses will be on the same page of a DRAM.